Naturally Circulating Hot Water System

Steve Greenberg is a facilities energy management engineer at Lawrence Berkeley National Laboratory and a technical editor at Home Energy.
Q: I have been having a discussion with a fellow worker concerning our house heating systems. He claims to have a natural circulating hot-water system, in a two-story row home. He says it has no circulating pump and that temperature differential was the only reason the system worked. I did some research and all the systems I have seen on the Net show them as open systems, not closed.
Also, the pipe size seems to have a lot to do with what creates the direction of flow. That is, large pipe for risers and small ones for returns. Isn’t it correct that for flow to occur, a differential pressure needs to exist? Isn’t this the reason for the different pipe sizes? Please explain how this system works. I find it hard to believe that temperature alone could move water through a boiler, piping, valves, and the radiators and back to the boiler.
Pete Ianuzzi
Baltimore, Maryland
A: I have been having a discussion with a fellow worker concerning our house heating systems. He claims to have ...